An inductor or a capacitor is used for phase matching of a high-frequency circuit. For example, there are demands for downsizing a RF (Radio Frequency) system such as a cellular phone or a wireless LAN (Local Area Network), reducing cost of the RF system, and enhancing performance of the RF system. For the same reason, downsizing, cost reduction and high performance are requested for a device mounted on the RF system. There is used an electronic device such as an integrated passive element in which a passive element is integrated.
There has been developed an integrated passive element in which a passive element is incorporated in a multi-layer ceramics substrate, with a LTCC (Low Temperature Co-filed Ceramics) technology as a method of satisfying the demands. There has been developed an integrated passive element in which a passive element is provided on a multi-layer ceramics substrate. There is, however, a problem that Q value of an inductor is reduced and a passive element having high Q value cannot be manufactured because relative permittivity of the ceramics substrate is higher than a quartz substrate.
Japanese Patent Application Publication No. 2007-123468 and Japanese Patent Application Publication No. 2007-31242 disclose an art where a coating layer is provided on a ceramics substrate and a passive element is provided on the coating layer.
A passive element such as an inductor is formed on an insulating layer having relative permittivity lower than that of a multi-layer ceramics substrate when the passive element is formed on the multi-layer ceramics substrate. This results in restraint of dielectric loss of the passive element. It is not preferable that a resin-based insulating layer such as PBO (Polybenzoxazole) or BCB (Benzocyclobutene) having low thermal resistance is used, because temperature of the multi-layer ceramics substrate is increased to a range of 200 degrees C to 300 degrees C in a manufacturing process of the passive element. It is preferable that an oxide layer having high thermal resistance is used. An opening is formed in the oxide layer in order to electrically couple a penetrating electrode and a passive element. Generally, the opening is formed in the oxide layer with a dry etching method using fluorine-based gas (for example HF, CF4 or the like) or a wet etching method using a solution including hydrogen fluoride. There is, however, a problem that etching speed of the dry etching method is low. And there is a problem that ceramics of the multi-layer ceramics substrate dissolves as well as the oxide layer when an etchant including fluorine or hydrogen fluoride is used in the wet etching method. It is preferable that the oxide layer is thick for the purpose of reducing dielectric loss of the passive element. However, etching time of the dry etching method is increased when the oxide layer is thick. On the other hand, the multi-layer ceramics substrate tends to dissolve because of side etching in the wet etching method.